The progress in developing an HIV vaccine is benefiting from a continuingly improved awareness of protective immune mechanisms and newly emerging vaccine vectors and concepts; there are encouraging results from animal model studies and human clinical trials. Progress in these areas, however, must be viewed with cautious optimism, since important questions continue to be asked about the relevance of certain animal models, the potency and breadth of immunity needed for efficacy, and the vaccine approaches most likely to succeed. New information is accumulating at a rapid pace, due in part to aggressive national and international vaccine initiatives. More than ever, it is essential to bring scientists together frequently for open and objective discussions of recent findings. The Keystone Symposium on HIV Vaccine Development, held in the spring, and the biennial NIH AIDS Vaccine Conference, held in the fall, are the only two major meetings devoted entirely to HIV vaccines in 2004. This Keystone Symposium meeting will cover a wide variety of topics ranging from natural SIV infection, human clinical trials, advances in immunobiology, HIV genetic variation, novel approaches to vaccination, and mechanisms of immune evasion. Graduate students and postdoctoral fellows will benefit from an opportunity to present their data in a highly interactive forum and from exposure to key scientific issues and stimulating discussion with knowledgeable investigators in a relaxed and informal setting. The goal of the meeting is to provide an attractive venue for scientists at all levels to share their recent findings, exchange ideas, and foster new collaborations that will build on recent advancements and continue to make significant progress toward a safe and effective HIV vaccine.